The present invention relates to the lighting arts. It is especially related to gallium nitride-based high power flip chip light emitting diodes for lighting applications, and will be described with particular reference thereto. However, the invention will also find application in conjunction with other light emitting diode applications, other types of light emitting diodes, and other types of semiconductor light emitters such as vertical cavity surface emitting lasers.
Light emitting diodes are increasingly being employed in outdoor displays and signals, indoor illumination, and other applications that call for high levels of light output. Many of these applications employ gallium nitride-based light emitting diodes that emit light in the blue to near ultraviolet range. For lighting applications, a suitable phosphor is typically applied as a die coating or is integrated into a die-sealing encapsulant to convert the blue or ultraviolet light emitting diode output to a white or other selected light. The power output of such light emitting diodes is determined by a number of factors, including: light extraction from the semiconductor die; lateral current uniformity across the die; and the effectiveness of die heat sinking.
In a flip chip arrangement, active light-generating layers are deposited on a light-transmissive substrate wafer, and frontside electrodes are formed on the light generating layers. The substrate wafer is diced, and each die is bonded to contact pads of a sub-mount, printed circuit board or other support in flipped orientation, that is, with the light generating layers proximate to the support and the substrate distal from the support. In the flip chip arrangement, light is extracted through the light-transmissive substrate. However, problems can arise in that wave guiding in the light-generating layers tends to trap light and reduce the light extraction efficiency. Moreover, uniformity of lateral current spreading across the active device area suffers because the n-type and p-type electrodes are non-overlapping. Heat sinking is also limited and asymmetric between the p-type and n-type electrodes.
The present invention contemplates an improved apparatus and method that overcomes the above-mentioned limitations and others.